This invention relates generally to ink jet apparatus, and more particularly to ink jet apparatus and methods of operating an ink jet apparatus in order to eliminate or at least substantially reduce problems associated with such apparatus during their start-up or utilization with pigmented inks.
The problems associated with ink jet start-up are legion and notorious. Among those problems, the most significant are misfiring or non-firing of the initial ink droplets, and slower initial ink droplet velocities. Such problems are generally believed to be a result of local change in ink properties resulting from phenomena such as water absorption from the air, chemical changes, or evaporation of the ink in the nozzle of the ink jet during an idle period between firings. Heretofore, such problems were addressed in a mechanical or electrical sense. That is, added pulses or signals were used to discharge the initial drop of ink in order to prevent misfiring, and to accelerate the ink drop to normal operating speed. Such approaches, however, nearly always involved complicated waveform shaping.
Other approaches to the alleviation of such problems, such as are disclosed in U.S. Pat. No. 4,400,215 and U.S. Pat. No. 4,537,631, each of which is assigned to the assignee of the present invention and is incorporated herein by reference, instead chose to address such problems in a chemical sense (i.e., formulating new inks in order to avoid problems associated with start-up). It is readily apparent, nevertheless, that such individualized approaches to the formulation of inks that are compatible with particular ink jet apparatus would ultimately lead to unnecessary and repetitive research and development for customized applications in order to eliminate or substantially reduce the problems associated with start-up of ink jet apparatus.
Another common problem encountered with ink jet apparatus involves their use with pigmented inks. That is, during periods of non-use, the pigments contained within the ink of such ink jet apparatus have a propensity for settling out or agglomerating. One approach used in the past to eliminate such settling was the incorporation of dyes in lieu of pigments within the ink. However, as is well known, pigments provide a much more intense color than their dye counterparts in typical inks use in an ink jet apparatus. It would, therefore, be desirable to provide an ink jet apparatus utilizing pigmented inks and method of operating the ink jet apparatus to reduce or at the very least substantially eliminate problems associated with start-up, and at the same time promote dispersion of the pigments within the ink through incorporation of acoustic microstreaming in the ink jet apparatus.
A great number of industrial applications of sonic or ultrasonic waves are known which create dispersions of particles in a liquid, or of liquid droplets in a gas. Also well known is the use of such waves to provide the reverse effect of causing agglomerations of particles in a liquid, or liquid droplets in a gas. The very fact that these exactly diametrically opposite effects can be achieved through the use of ultrasonic energy indicates that at least more than one mechanism must be at work. In fact, several mechanisms have been identified that could account for the motion of suspended particles in a sound field. To consider what role these mechanisms play in an ink jet apparatus, it is convenient to classify them into three groups: (1) forces associated directly with the oscillatory motion of the sound field; (2) cavitation activity; and (3) acoustic microstreaming.
For all acoustic waves of practical amplitudes and frequencies, the particle displacement amplitude is extremely small. As an example of this, it can be shown that the particle displacement amplitude, a, for a plane wave in a liquid of density, .rho., and sound velocity, c, having a pressure amplitude, p, and a frequency, f, would be: ##EQU1## For an exemplary ink jet apparatus in which p=10.sup.6 dynes/cm.sup.2, =1 gram/cc, c=1.5.times.10.sup.5 cm/sec, and f=50 kHz, the particle displacement amplitude, a, would be on the order of approximately 2000 angstroms. Solid particles in suspension within the liquid would, therefore, have an oscillatory motion of 2000 angstroms or less. In general, heavier denser particles would undergo an oscillatory motion having much smaller amplitudes.
Associated with these motions are weak interparticle forces, and an increase in the probability of particle collisions which could lead to agglomeration. This type of mechanism has been demonstrated especially well in gases, and very often has been shown to occur in the presence of a standing wave where the resulting weak forces lead to a slow migration of the particles towards nodes or antinodes (depending upon the relative density of the particles). It can be readily appreciated therefore that, even in such a relatively simple case of a standing wave, the mechanisms involved which can result in weak forces acting on particles in suspension can be quite complicated, especially in the case where boundaries in the form of liquid/solid or liquid/air interfaces are introduced.
Another class of forces which are much stronger than forces associated with oscillatory particle motion as discussed herein above, and which are still associated with an ultrasonic wave in a liquid, are those forces resulting from cavitation activity. The most violent forces are associated with vaporous cavitation which occurs in a liquid when voids or cavities are produced in the liquid during the negative half cycle of the sound pressure wave. Cavities formed in this way collapse violently during the subsequent positive half cycle, and result in the production of a microscopic shock wave with very high pressures and temperatures. These conditions, which are typically present within an ultrasonic cleaning bath, can readily result in the breaking up of agglomerations with subsequent dispersion of the particles in a liquid. Below this threshold, however, there is another cavitation phenomenon known as stable cavitation.
Although less violent than vaporous cavitation, stable cavitation can also result in relatively large forces which may act on particles in suspension. Stable cavitation is generally associated with gas bubbles which already exist in the liquid, or which grow from dissolved gas coming out of solution under the action of the sound wave. A gas bubble in such liquids has a very high mechanical Q factor, and hence at resonance, the amplitude of motion can rapidly build up to very high levels. When this occurs, a variety of non-linear effects occur in the vicinity of the bubbles including bubble break-up and large pressure gradients in the liquid immediately surrounding the bubbles. A phenomenon known as acoustic microstreaming also occurs in the vicinity of such an oscillating bubble and can, of itself, contribute to the ultrasonic dispersion and breaking up of agglomerates in a liquid.
Acoustic microstreaming is also a non-linear effect, but one which can occur at amplitudes well below the threshold for vaporous cavitation. Although generally associated with nonlinear liquid/air oscillations, there are also situations when bubbles are not present where vigorous microstreaming can occur. Acoustic microstreaming is a steady, non-oscillatory flow of the liquid on a very small scale, usually taking the form of microscopic eddies which can be pictured in a somewhat simplistic manner as the flow resulting from small scale radiation pressure gradients. Such radiation pressure gradients can be found around regions where a sharp discontinuity exists, such as at the tip of a vibrating rod having a radiating surface the dimension of which is very small as compared with its wavelength of vibration. Radiation pressure gradients may also be found around other types of geometrical discontinuities (e.g., corners or edges) of solid surfaces in contact with the liquid.
In general, acoustic microstreaming results in a small scale stirring action in the liquid, the physical and chemical effects of which are well documented within the prior art. For example, the stirring action around the tip of a vibrating needle has been visualized by immersing the vibrating needle in a dilute solution of photographic developer just above a piece of partially exposed photographic paper, the image developed on such paper clearly showing microstreaming flow lines. The action of microstreaming in stirring the inside of living cells has also been suggested as the mechanism which explains many of the biological effects of low amplitude ultrasonic radiation. In spite of such suggestions, however, the inventors herein know of no ink jet apparatus which incorporates a means for acoustically microstreaming to eliminate or at the very least substantially reduce problems associated with their start-up, or to maintain a dispersion of pigments employed in pigmented inks.
One means and method of operating an ink jet apparatus to reduce start-up problems is disclosed in U.S. Pat. No. 4,323,908, issued to Lee et al. The Lee et al. device purges any entrapped air from the ink cavity and nozzle orifice of the print head of a drop-on-demand ink jet printer by energizing a tubular piezoelectric transducer with a series of pulses for a preselected short time period and at a repetition rate substantially equal to a resonant frequency of the ink cavity. Except during purging, the transducer operates asynchronously in drop-on-demand mode in response to discrete binary print signals.
While completely silent as to its applicability for acoustically mixing a pigmented ink, the Lee et al. device nevertheless utilizes a sinusoidal excitation of the drive transducer during non-printing periods for the purging of entrapped air form the ink cavity and nozzle orifices. However, the Lee et al. device has an extremely narrow range of operation around the frequency of device resonance. Moreover, it is evident from the teachings of Lee et al. that a stream of ink must be ejected from the nozzle orifices during purging of air therefrom, again since the device must operate at a resonance. As a result, incorporation of such a device in an ink jet printer requires a complicated head tending system to ensure the removal of excess ink purged along with the air.
Another device designed to prevent the precipitation of ink and lacquer suspensions during the operation of writing systems, especially ink jet writing systems, is disclosed in German Specification (i.e., "Offenlegungsschrift") No. 3,508,389, published Sept. 11, 1986. The device as disclosed therein, unlike the device of the above described U.S. Pat. No. 4,323,908, is adapted for pulse-type operations (i.e., always when the writing head is not in operation such as during the writing interval or carriage return) and does not release any more droplets during the writing intervals, but is adequate for the blending of the fluid.
In such a device, one or more crystal units are mounted on the ink reservoir and/or the writing grooves of a recording mechanism as described in Siemens-Zeitschrift, Volume 4, April 1977, pages 219-221. Such grooves are concentrically enclosed by transducers which contain piezoceramic tubules the energy of which, according to German Specification No. 3,508,389, is lowered during the writing intervals such that it does not release any more droplets, but is adequate for the blending of the fluid. As such, the device of the above described German Specification avoids the problems associated with the device of U.S. Pat. No. 4,323,908 in that no head tending apparatus is required for the ink which is purged from the nozzle orifices. However, such a device is limited in use with ink that does not dry up in the nozzle openings. Moreover, the device of German Specification No. 3,508,389 has a low operating frequency, and, because of the writing grooves' being long as compared with the wavelength of the transducers operating at resonance is capable of setting up standing waves, but nearly incapable of producing microstreaming (i.e., to provide the larger local intensity gradients which are required for microstreaming).